The present invention relates to large diameter casings for the oil and gas industry having one or multiple thread on a conical end the number of threads per inch pipe or tube length determining the thread as a whole.
Tubes, pipes and casings in the oil and gas industry are usually constructed with a conical end (pin member) which is threaded so that pin and box ends of two such tubes, casing or pipes can be interconnected directly or pin member (male ends) of two such tubes, casings or pipes can be interconnected by a threaded sleeve. The API standards for such casings require that the number of threads and the tapering of the end cone be uniform over the entire range of interest. This has the advantage of a rather economic cutting tool inventory for making such threads. However, this requirement is definitely a disadvantage when taking the use of such a pipe or casing into consideration; the dimensions of the thread are determined by the smallest diameter and the matching wall thickness. The smallest diameter usually being 41/2 inches. In practice, however, particularly when used in a drilling derrick, pipes or casings having such small thread dimensions require extremely exact and sensitive handling. These requirements and precausions are actually inordinately extensive in view of the large pipe diameter and the very large weight of the casing so handled, even at diameters of from 300 to 400 millimeters which are not uncommon. It was observed, however, that pipes, having a diameter in excess of 500 millimeters, 5 threads per 1-inch thread length and a thread cone corresponding to a taper of 1 to 12, cannot be threaded together because even a small and imperceptable skew angle between the two pipes to be threaded together prevented proper thread engagement and interthreading. As a consequence, the threaded connection was not capable of taking up the requisite load and was even damaged to some extent.